Automatic frequency control circuit



' Dec. 16; 1941.

N. M. Ru sT EFALY 2,266,517 AUTOMATIC FREQUENCY CONTROL CIRCUIT FiledJa n, -e, 1940 2 Sheets-Sheet 2 INVENTOR NOEL M. RUST s Og WOLD E- K ALL 'M'v-a,

A TTbRNEY Patented Dec. 16, i941 2 2 517;

AUTOMATIC FREQUENCYCDNTROL CIRCUIT N 061 Meyer: Rust, h msfiord, and; QSW d. Edward. Keall, .West Hanningfield, England, as-. signors tov Radiocprporation offAmeric'a a corporation of Delaware Applicationnnuary-rs, 1910;.Serial No..312 ,6614

n Great Britain Mana ua-.1933.

11'Glaims; 21.5.amaze) invention; relatesto; automaticz frequency systemamaw be eefiectedemanuallyi. quafi-autor control systems suitable for vuse:inxradiaaand like; maticallwonwholly automatically. Eorezgample, receivers. V ionmanual:.varia.tionv .(which. is ,notpreierred in Automatic, frequency control systems;- .1a1;e 1 new practice.) .1; there would. be provided amanually in;c.ommon use in radioreceivers, butsuch:sy -s operableswitch:whichvariesthe sharpness of the temsasare at presentknown .fall shorko system;.in; oneor.;.mo.re.steps,.and which quirementsin certain. importantoperatingpar: should beamoved'from. a broad: position, ,to a ticularsi As.will.beappreciatedamain.purpose. sharp? position when-the manual; tuning;oper of, an; automatic frequency: control, hereinafter; ation .iscompletem. In. aquasirautomat c ys-r termed..(AFC), system .is to .take; charge of-the. 10 tem;.the control. of; the,- sharpness of; the-,AFC-

tuningof the receiver. once;.it .hasbeemmanuallm system= Would..be;-.eifected-; by. a;switch- .;op erated tuned to within a predeterminededegree: o:-..ac: automatically-1mm. the. normally pr.ov.ided; tuning curacy so that the necessity. for; skilledituning handlelof:thelreceiver;sotthatzwhen, said-tuning on the. part of theoperator.ofthereceiven is me handle -11s;beineppera edz hesw tcb. autm at duced,.or eliminated. Obviously from the; point 175; call-y; si'lt'i flf 1H S Q -.g V g .b oad? a of view of the extent towhichzskilledzmanual tion;theswit hposition chaneinsa at al r tuning is rendered unnecessaryitgisi desirable. to. one;giving sharp, actionwhen-the manual that the-AFC' system shall: take charge when: tuningz zoperation iszc mpl te Switches dap the -manual tuning has. beenv set: quite: a long to besautomatically;operated byatuning Control way.- from the accurate tunings position. This; zo handlewareg of... course, known per Se.v re requirement, however, 1 .;inconsistentwith: the purposes It. isprelermd, however; to;err 1-p loyv maintenance of closely. accurate: tuningaby the? fully automatic;variation of'.the;AFC sha llnessv AFC.system, and, accordingly, in known and-ginasystemof this nature theactual-c ntr systems a compromise :hasbeen. sought; the onoutput,zvoltage;.fromthe.AFG: system isemsharpness of the system being-.chosen-at' some 25. ployed to; vary; the; sharpness, of said 'systemi.

value whichis not so great as to make-theasets Preferably; Particularly; in fully: automaticsys; difficult to tune by an unskilled operatorybuti ternsinaccordance.with.thisinyention;the1 ar- Which isnevertheless great enoughtO'ug'iVG a; rangementiis made-.suchthat theselectivity of reasonable degree ofaccuratetuning. the-.receiveris automatically variedzat-the, samev sharpness in a known- AFC- system isialcom- 3 time-asthe :sharpnessof theAFQ system sothat,

promise, and, therefore, is not wholly satisfactory: the receiver: becomes. more. selective 1 as .the AEC Itisbelieved that none of theknownAECsysteins system becomes sharper. I

is-of sufficient sharpness to obtain and main- The. invention; illustrated inwthe accomtain really accurate tuning, for example tuningi panyingi drawings, wherein accurate enough for a homodyne receiver-. 35 Figs. liand Zshow; two .embodimentso-f the. in;-

According to this invention arad-io, or-lik'e, vention:

receiver is provided With-an AFC system of-"vari- Figure l1illustratesone wayof carrying out thisable sharpness so that afairly broad AFG inventioniasapplied.to;.a homodyne receiver. A.

action is obtained when tuning inasta-tion; and valve V1to-which received carrier. and;side;.ba-nd:

a sharp action is obtained at othertimes once signalsareapplied, hasin its cathode leg.:a.f.eed.-. the- AFC' system has takencharge It back impedance Z 'of;any. suitable frequencysethought that the expressions broad and sharp lective typei Preferably, and as shown, thedmas applied to an AFC systemwill-be found fairly pedance consists. of: a piezo-electric crystal-j C,

self-explanatory, butit may-rbe remarked: that an resonant at thelcarrier frequency. inv shuntwith- AFC system may be said to be sharp when lar-ge 453 an inductance- L- and: with. a. condenser. K1, the,

tuning control output: voltages are provided for combination being adjusted to neutralize, the short departures from correct tuning; .whileshunt capacity: of said crystal. The received. conversely, the system is broad-if a large decarrier and side band signals are applied to.thev parture f-rcmcorrect tuning is required to-procontrol grid [10f valve .V, and the said control,

vide a large control voltage. It will be appre-s 50 grid is connected'throughtwo resistances RrR z.

ciated that it is inherent inanyAFC systemthat: in-seriestoearth, the second resistance.-.thatvon if'-thesystem is sharp the range over which it the ground side.being-shunted by. a condenser takes charge'is small; whereas if it-is -broad athe K2; The junctionpoint of these twolresistances.

range-=overwhich=it takes' -charge islarge: r is connected? to one of apair of: contacts-.of-a. The variationor-the" sharpness of the switchS;-.the:other contactgofiwhichisconnected;

to a point of negative bias potential, this point being connected to ground through a suitable resistance, or as shown derived from a battery having its positive terminal earthed.

The switch is operated in any convenient manner known per se as by a cam CA on the actuating shaft of the normally provided tuning condenser gang shaft so that when the said shaft is moved tuned circuits in series one resonant a little above the operating I. F. and the other a little below the operating I. F.

In the above described specific embodiments the change in sharpness of the AFC system is obtained by varying the mutual conductance of a valve to which frequency selective feedback is applied. This, of course, is not a necessary feain manual tuning the switch is closed. Output which may contain chiefly I. F. (intermediate frequency) carrier component is taken from the anode of the valve, and is utilized as in the usual which rectified AFC voltage is derived for ap.v

plication, as in the usual well known way, to control the mutual conductance of a frequency control tube connected, in the usual manner, to the local oscillatortube. The mid-point of the resistance in the diode network is, of course, connected to the mid-point of the inductance therein, and input signals consisting of I. F. carrier and side bands are applied to these two midpoints as well as to the input grid of the valve.

It will be seen that with this arrangement, in one position of the switch, the degenerative crystal feedback network is practically ineffective to produce feedback effect by reason of the high bias applied to the grid of the valve, but in the other position of the switch the feedback network is effective and a substantial amount of negative feedback is provided. Accordingly the selectivity,

or sharpness, of the valve circuit isvery different for the two positions of the switch, being largest for the least bias. Change over from one son of the fact that when the switch is opened, after having been closed, the bias on the valve does not change suddenly but only smoothly during the period in which the charge in the condenser across the second of the two resistances in the grid circuit leaks away.

It will be seen that with this arrangement not only are two widely different degrees of sharpness of AFC action obtained, one being suitable for searching and the other being suitable for maintaining accurate tuning'once a station has been found, but, also, in the case of homodyne reception the selectivity of the receiver itself is simultaneously varied. This action is obtained by reason of the fact that the valve which is in the AFC discriminator network, 1. e. the valve V with the crystal network in its cathode leg, is also the valve which is employed to provide the carrier component for homodyne action.

Although the use of a crystal network is no an essential in carrying out this invention, its use is preferred for the homodyne case where a very high degree of selectivity and a very high degree of AFC sharpness is necessary. In many cases however, and especially in broadcast superheterodyne receivers, it may be found commercially more economical to use in place of a crystal feedback impedance in the cathode leg of the valve, a feedback impedance consisting of two ture of the invention for the feedback could be varied in other ways, for example, by providing a variable resistance across the crystal network in the first described arrangement and altering the value of this resistance by the switch.

Again, as already stated, the variation of sharpness of the AFC system and, if desired, the simultaneous variation of selectivity of the receiver may be accomplished entirely automatically. In broad principle this may be effected by deriving from the AFC system a potential which is dependent upon the magnitude of departures from correct tuningbut independent of the senseof such departures and utilizing the said voltage to control the sharpness.

1 Thus, in the first embodiment herein described the 'switchand-associated circuit elements could be dispensed with and instead there could be employed a rectifier bridge B as shown in Fig. 2, having the opposite ends of one diagonal connected between the cathodes of the diodes D'1--D2 in theAFC rectifier network. One end of the other diagonal is connected through a resistance-to the low potential end of the signalinput circuit of the valve V and the other end .of said diagonal is connected to ground. With; this arrangement, of course, the grid bias on the .valve, andwtherefore the effectiveness of the frequency selective impedance in the cathode leg of the valve. for feedback, would be made directly dependent upon the magnitude of thereotified output voltage obtained from the AFC. networkzbut independent of the polarity of that voltage. Thus a change in the magnitude of the derived AFC.output voltage produces a change inv the biasof the valve V in the discriminator network, and, therefore, a change in the AFC sharpness; Inthis case, as in the previous embodiments, if the valve in the AFC discriminator network were also a valve in the signal channel simultaneousvariationof AFC sharpness and of selectivity would be obtained. The diodes D1D2 function as the rectifiers for producing AFC bias, as in thecase of Fig. 1. i The bias delivered over line S: increases in proportion to the departure from correct tuning in either direction, thus lessening theyselectivity and giving a broader AFC control. As the signal is brought into tune the bias over S decreases and selectivity increases sov thatthe AFC action is sharper and holds the tuningvery close.

The invention is, of course, not limited to the use of variable negative frequency selective feed:

back; in order, to obtain variation in sharpness, for variable positive feedback circuits, of any,

applied carrier energy whose magnitude and po-- larity is a function of the extent and sense of carrier frequency.- change, andmeans-connected to said discriminator input circuit responsive to a: departure: of said carrier: frequency from-apredetermined frequency: value for controlling the selectivityof' said"transmission path; 1

2. In. combination. with awsource. of. modulated carrier energy; a transmissionipath for. said ener gy coupled tosth'e source; saidipath. being selectiveto energyof 'said carrier'frequency, a frequency discriminator networkcomprising. an input circuit coupled to said. path, said network having an output circuit... constructed and arranged to develop a. direct:v current. voltage from applied carrier? energy; whose magnitude and pol'arity isha.function/of theextent and. sense of carrier frequency :change, and means. responsive to a departure of said carrier frequency from a predetermined;gfrequency, value for controlling the;selectivity-of'said transmission path, said last means comprising a rectifier network whose input circuit is the said discriminator input circuit.

3. In combination with a source of modulated carrier energy, a transmission path for said energy coupled to the source, said path being selective to energy of said carrier frequency, a frequency discriminator network comprising an input circuit coupled to said path, said network having an output circuit constructed and arranged to develop a direct current voltage from applied carrier energy whose magnitude and polarity is a function of the extent and sense of carrier frequency change, and means responsive to a departure of said carrier frequency from a predetermined frequency value for controlling the selectivity of said transmission path, said discriminator comprising a pair of rectifiers, and said last means comprising a pair of rectifiers having a common input circuit coupled to said path.

4. In an automatic frequency control system for a super-heterodyne receiver of the type comprising a first detector, a local oscillator, an intermediate frequency network coupled to the first detector, means coupled to said intermediate frequency network, responsive to a frequency difference between the intermediate frequency signals and a predetermined frequency value, for adjusting the local oscillator frequency in a sense to correct for said difference; the improvement which is characterized by said intermediate frequency network being selective for said intermediate signals, and means coupled to the same point of said intermediate network as said first means, and responsive to said intermediate signals, for decreasing the selectivity of said intermediate network during periods of said frequency difference.

5. In an automatic frequency control system for a super-heterodyne receiver of the type comprising a first detector, a local oscillator, means for tuning the detector and oscillator over a range of signal frequencies, an intermediate frequency network coupled to the first detector and means, responsive to a frequency difierence between the intermediate frequency signals and a predetermined frequency value, for adjusting the local oscillator frequency in a sense to correct for said difference; the improvement which is characterized by said intermediate frequency network being selective for said intermediate signals, and means concurrently operative with the tuning means for decreasing the selectivity of said intermediate network during periods of said frequency difference, said intermediate network circuits,;means commomto; said outputand input circuits: for. PIOVidlIlQII degenerative. feedback therebetween}. and said; selectivity decreasing meansbeingarrangedto; control the gain of said tube:

6.:.:In-an. automatic frequency controlsystem for: alsuper -heterodyne. receiver. of the. type com.- prising afirstzdetector, av localoscillator, anintermediate; frequency. network; coupled. to. the first;;detector and; means, responsive to a frequency: difference. betweenv the intermediate frequency: signals; and a. predetermined. frequency value-,1; for-adjusting the. local. oscillator frequency in;;a senseitolcorrect for said difference; the improvement which. is. characterized by said intermediateefrequency network being selective for saidintermediate signals; and means for decreasing; the selectivity .of: said intermediate, network during periods of said frequency difference, means. fcnadjusting. the: tuning of said first. detector and oscillator over a desired modulated carrier range, and means for adjusting said selectivity decreasing means concurrently with said tuning means.

I 7. In an automatic frequency control system for a super-heterodyne receiver of the type comprising a first detector, a local oscillator, an intermediate frequency network coupled to the first detector and means, responsive to a frequency difference betwen the intermediate frequency signals and a predetermined frequency value, for adjusting the local oscillator frequency in a sense to correct for said difference; the improvement which is characterized by said intermediate frequency network being selective for said intermediate signals, and means for decomprising a tube having signal input and output creasing the selectivity of said intermediate network during periods of said frequency difference and additional means for simultaneously adjusting the tuning of said receiver and said selectivity decreasing means.

8. In a radio receiver of the type having a main tuning means for selecting different carrier frequencies of a modulated signal carrier frequency range, a supplemental tuning means, responsive to a frequency difference existing between received modulated carrier energy and a predetermined carrier frequency value, for adjusting the receiver tuning in a sense to correct for said frequency difference; the improvement which is characterized by said supplemental means having a relatively sharp action at settings of said main tuning means at which said difference is a minimum, means for broadening said action in response to said difference exceeding said minimum and means for decreasing the receiver selectivity concurrently with actuation of said broadening means.

9. In a radio receiver of the type having a main tuning means for selecting different carrier frequencies of a modulated signal carrier frequency range, a supplemental tuning means, responsive to a frequency difference existing between received modulated carrier energy and a predetermined carrier frequency value, for adjusting the receiver tuning in a sense to correct for said frequency difference; the improvement which is characterized by said supplemental means having a relatively sharp action at settings of said main tuning means at which said difference is a minimum, and means for broadening said action in response to said difference exceeding said minimum and means for simultaneously varying the selectivity of the receiver in the same sense as said supplemental tuning action.

10. In a radio receiver of the type having a main tuning means for selecting different carrier frequencies of a modulated signal carrier frequency range, a supplemental tuning means, responsive to a frequency difference existing between received modulated carrier energy and a predetermined carrier frequency value, for adjusting the receiver tuning in a sense to correct for said frequency diiference; the improvement which is characterized by said supplemental means having a relatively sharp action at settings of said main tuning means at which said difference is a minimum, means, responsive to said modulated carrier energy,. for broadening said action in response to said difference exceeding said minimum and additional means responsive to said carrier energy for decreasing the receiver selectivity conjointly with actuation of said broadening means.

11. In a radio receiver of the type having a 20 main tuning means for selecting different carrier frequencies of a modulated signal carrier frequency range, a supplemental tuning means, responsive to a frequency difference existing between received modulated carrier energy and a predetermined carrier frequency value, for adjusting the receiver tuning in a sense to correct for said frequency difference; the improvement which is characterized by said supplemental means having a relatively sharp action at settings of said main tuning means at which said difference is a minimum, means for simultaneously decreasing the receiver selectivity and broadening said action in response to said difference exceeding said minimum, and means for simultaneously adjusting said main tuning means and said last means. 7

NoiiL MEYER RUST. OSWOLD EDWARD KEALL. 

